Add first_run.sh

[RSDK-9279] Add first_run.sh to pull docker image (#25)

Things I have tried:
- `make module.tar.gz` constructs a shell script using the correct tag, and includes the shell script in the .tar.gz file.
- Running that shell script starts downloading docker images. I got impatient and stopped it before it finished

Things I have not tried:
- Running the module through `viam-server` to see if it invokes `first_run.sh` the way it should. If it's a local module, it's unclear to me how `viam-server` knows about `first_run.sh`, and if it's not a local module, I need to upload to the registry first.

Unrelated to the main change: I added a `set -x` (print out every command in the shell script before you run it), for ease in debugging stuff. 


* add 'set -x' at the top for help debugging, and add comment on 'exec'

* add first_run.sh template

* build and use first_run.sh

* generalize the envsubst rule

* whitespace tweak